This invention relates to methods for testing electrical devices, and more particularly to a testing arrangement for printed circuit boards or the like.
Electrical components such as printed (or etched) circuit boards must be tested after manufacture has been completed. The testing is primarily directed to determining whether or not all of the conductive paths are in their designated positions and no conductors are unintentionally shorted, and whether electrical continuity exists where it should. Various functional tests may be executed using the external connectors (those included for operation in the end equipment), and also visual inspection can provide an indication of the quality of the part. Such methods can be time-consuming and expensive, however, and in any event are not always effective in discovering shorts or opens which may be hidden form view or not exercised in the electrical functional tests chosen. For these reasons, various other test mechanisms have been proposed which perform the function of scanning the surface of a etched circuit board and providing an indication of the wiring integrity.
Electron beam technology is used to scan an integrated circuit chip or an etched circuit board under test while detecting secondary electrons emitted by the pattern on the board. One of these methods is referred to a Voltage Contrast Electron Beam (VCEB) technique; these testers are like logic analyzers which probe functional electronic circuits on semiconductor integrated circuit chips. The VCEB technique is described by Woodard et al, J. Vac. Sci. Technol., Nov./Dec. 1988, p. 1966, "Voltage Contrast Electron Beam Testing Experiments on Very Large Scale Integrated Circuit Chip Packaging Substrates". When used for testing relatively large devices, the VCEB technique requires complex and expensive electron beam deflecting arrangement, as well as requiring a large evacuated chamber.
Another testing method using a scanned E-beam is described in Abstract No. 129, Extended Abstracts, The Electrochemical Society, Spring Meeting, May 10-15, 1987, Vol. 87-1, p. 185, by Paul May et al, "Laser Pulsed E-Beam System for High-Speed I.C. Testing"; May et al describe an instrument for non-invasive testing using a laser beam impinging upon a gold-coated quartz photocathode to produce an electron beam, but again the electron beam must travel a large distance and must be focused and scanned by some intervening mechanism.
A laser beam is used instead of an electron beam in a test method shown by A. M. Weiner et al, "Picosecond Temporal Resolution Photoemissive Sampling", Applied Physics Letters, May 1987. Emission of electrons from a part under test at the point where the laser beam impinges is detected. A similar test method is shown by R. B. Marcus et al, "High-Speed Electrical Sampling by fs Photoemission", Applied Physics Letters, Aug. 11, 1986, p. 357, where a method for contactless probing of high-speed electrical waveforms, by spectral analysis of the photoelectrons emitted when a signal-carrying electrical conductor is illuminated by ultrashort laser light pulses.
In a copending application Ser. No. 424,624, filed Oct. 20, 1989 by Ellsworth W. Stearns, for "Single-Probe Charge Measurement Testing Method", assigned to Digital Equipment Corporation, a testing method is disclosed which uses the charging and discharging of nodes on a printed circuit board or the like by a probe physically touching the nodes. The current resulting from this charging is observed to determine whether the correct X-Y pattern is exhibited for charging these nodes.